Composite skin and stringer structure and method for forming the same

ABSTRACT

Composite stringer and skin structures and methods for forming the same are disclosed. In one embodiment, a composite stringer and skin structure includes a polymer-based elongated stringer portion having reinforcing fibers positioned in a plurality of adjacent plies, a first portion of the reinforcing fibers being oriented at a relatively shallow angle relative to a selected reference direction, and a second portion of the reinforcing fibers being oriented at a relatively broad angle relative to the selected reference direction. A polymer-based and fiber reinforced skin member adjoins the stringer portion, and an adhesive material is interposed between the stringer portion and the skin member.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is related to the following co-pending,commonly-owned U.S. patent applications, which applications are herebyincorporated by reference: U.S. patent application Ser. No. 11/096,743entitled “Composite Structural Member and Method for Forming the Same”,filed under Attorney Docket No. BING-1-1151; U.S. patent applicationSer. No. (to be determined) entitled “Hybrid Fiberglass CompositeStructures and Methods for Forming the Same”, filed under AttorneyDocket No. BING-1-1149; U.S. patent application Ser. No. 11/096,727entitled “Multi-Axial Laminate Composite Structures and Methods ofForming the Same” filed under Attorney Docket No. BING-1-1150; U.S.patent application Ser. No. 11/096,495 entitled “Composite StructuralMember Having an Undulating Web and Method for Forming the Same” filedunder Attorney Docket No. BING-1-1133.

FIELD OF THE INVENTION

This invention relates generally to composite structural components, andmore particularly, to a composite skin and stringer structure.

BACKGROUND OF THE INVENTION

Structural members are available in a wide variety of configurations toprovide structural support under a variety of loading conditions. Inparticular, the wing and empennage surfaces of an aircraft typicallyinclude parallel and span-wise oriented structural members calledstringers that are coupled to skin members on the wing and empennagesurfaces that cooperatively provide the desired flexural and torsionalstiffness to the wing and empennage surfaces. Typically, the wing andempennage surfaces are fabricated from a metal, such as aluminum, steelor titanium, and the stringer includes a planar web portion that isgenerally oriented in a direction approximately perpendicular to theskin member and extending in a span wise direction along the wing orempennage surface so that the web portion offers resistance to a bendingmoment generated by the load. A flange portion may be positioned on oneor both of the longitudinal edges of the web portion in order to provideresistance to localized failure of the web portion due to lateralbuckling. The flange portion further allows the stringer to be coupledto the skin member by providing an attachment surface for the skinmember.

Reinforced polymer-based materials are also available that may be usedto form various structural members, and may be used as a substitute formetals, particularly in applications where relatively low weight andhigh mechanical strength is desired. As a result, reinforcedpolymer-based materials are widely used in a variety of commercial andmilitary aircraft, terrestrial vehicles and consumer products. Thematerial is generally comprised of a network of reinforcing fibers thatare generally applied in layers, and a polymeric resin thatsubstantially wets the reinforcing fibers to form an intimate contactbetween the resin and the reinforcing fibers. The material may then beformed into a structural component by a variety of known formingmethods, such as an extrusion process or other forming processes.

Although desirable results have been achieved using prior art apparatusand methods, a stringer and skin structure that may be more easily andinexpensively fabricated, and that may provide a more favorable strengthto weight ratio in comparison to conventional stringer and skinstructures, would have utility.

SUMMARY

Composite stringer and skin structures and methods for forming the sameare disclosed. In one embodiment, a composite stringer and skinstructure includes a polymer-based elongated stringer portion havingreinforcing fibers positioned in a plurality of adjacent plies, a firstportion of the reinforcing fibers being oriented at a relatively shallowangle relative to a selected reference direction, and a second portionof the reinforcing fibers being oriented at a relatively broad anglerelative to the selected reference direction. A polymer-based and fiberreinforced skin member adjoins the stringer portion, and an adhesivematerial is interposed between the stringer portion and the skin member.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed embodiments of the present invention are described indetail below with reference to the following drawings.

FIG. 1 is an exploded, partial cross sectional view of a composite skinand stringer assembly according to an embodiment of the invention;

FIG. 2 is a schematic view of a ply arrangement for the skin member ofFIG. 1, according to another embodiment of the invention;

FIG. 3 is a schematic view of a ply arrangement for the stringer portionof FIG. 1, according to still another embodiment of the invention;

FIG. 4 is a schematic view of a ply arrangement for the stringer portionaccording to yet another embodiment of the invention;

FIG. 5 is a flowchart that shows a method of making a composite stringerand skin structure according to still yet another embodiment of theinvention; and

FIG. 6 is a side elevation view of an aircraft having one or more of thedisclosed embodiments of the present invention.

DETAILED DESCRIPTION

The present invention relates to composite skin and stringer structuresand methods for forming such structures. Many specific details ofcertain embodiments of the invention are set forth in the followingdescription and in FIGS. 1 through 6 to provide a thorough understandingof such embodiments. One skilled in the art, however, will understandthat the present invention may have additional embodiments, or that thepresent invention may be practiced without several of the detailsdescribed in the following description. In the present discussion, it isunderstood that the term “reinforced polymer-based material” includesvarious non-homogeneous polymer-based materials, commonly referred to as“reinforced composites”, “carbon-fiber composites”, or still other termsknown in the art.

FIG. 1 is an exploded, partial cross sectional view of a composite skinand stringer assembly 10 according to an embodiment of the invention.The composite skin and stringer assembly 10 includes an elongatedstringer portion 12 having a web portion 14 that is positioned between afirst flange portion 16 and an opposing second flange portion 18. Theweb portion 14 may have a predetermined depth D in order to provide adesired resistance to an applied loading. The first flange portion 16and the second flange portion 18 are generally planar members havingpredetermined widths W₁ and W₂, respectively. The web portion 14 and thefirst flange portion 16 and the second flange portion 18 may be constantalong a span of the stringer portion 12 (i.e. into the page), or theymay vary continuously, or even non-continuously along the span of thestringer portion 12. The web portion 14 and the first flange portion 16and the second flange portion 18 are generally formed from a reinforcedpolymer-based material having multiple layers of reinforcing fibersoriented in a predetermined orientation. The orientation of thereinforcing fibers in the multiple layers will be described in greaterdetail below.

The assembly 10 also includes a skin member 20 having a desiredthickness t₁ that is coupled to the second flange 18 using, for example,a suitable adhesive material. In general, one of the stringer portion 12and the skin member 20 may be in a relatively uncured state, while theother may be in a relatively cured state. For example, and in oneparticular embodiment, the skin member 20 may be coupled to the secondflange 18 by interposing a film adhesive material between a relativelyuncured skin member 20 and the second flange 18 of a relatively curedstringer portion 12. The film adhesive is then cured while the uncuredskin member 20 is cured, thus forming an adhesive bond between thesecond flange 18 and the skin member 20. Alternately, the film adhesivemay be interposed between a relatively cured skin member 20 and arelatively uncured stringer portion 12, so that the adhesive bond isformed while the stringer portion 12 is cured. One suitable filmadhesive is the FM-300 film adhesive, available from Cytec Industries,Incorporated of West Paterson, N.J. although other suitable alternativesexist. In another particular embodiment, the stringer portion 12 and theskin member 20 may both be in a fully cured state, and a suitable pasteadhesive may be interposed between the second flange portion 18 and theskin member 20. The skin member 20 is also generally formed from areinforced polymer-based material having multiple layers of reinforcingfibers oriented in a predetermined orientation. The orientation of thereinforcing fibers in the multiple layers will also be described ingreater detail below.

Still referring to FIG. 1, the skin member 20 and the second flangeportion 18 may include a least one set of corresponding apertures thatproject through the skin member 20 and the second flange portion 18. Theapertures are suitably sized to accommodate a fastener 22 thatthreadably engages a nut portion 24 that cooperatively couple the skinmember 20 and the second flange portion 18. The fasteners 22 and the nutportions 24 further cooperatively impart a predetermined compressiveforce to the skin member 20 and the second flange portion 18 when apredetermined torque is imparted to the fastener 22. In one particularembodiment, the fasteners 22 and the corresponding nut portions 24 maybe selected from the well-known National Aerospace Standard (NAS) seriesof structural bolts and corresponding nut portions, although otheralternative fasteners exist. For example, the HI-SHEAR, HI-LOK orHI-LOK/HI-TIGUE fastening systems available from the Hi-ShearCorporation of Torrance, Calif. may also be used to couple the skinmember 20 and the second flange portion 18. The foregoing fasteningsystems develop a predetermined tension by including a shear portionthat breaks during installation when the predetermined tension isachieved. Consequently, a torque inspection following installation isunnecessary. The one or more fasteners 22 may advantageously reinforcethe adhesive bonding between the second flange portion 18 and the skinmember 20, and further may advantageously provide a crack-arrestingfeature.

FIG. 2 is a schematic view of a ply arrangement 30 for the skin member20 of FIG. 1, according to another embodiment of the invention. The plyarrangement 30 includes a plurality of interleaved plies that furtherinclude generally parallel reinforcing fibers. Accordingly, the plyarrangement 30 includes a first layer 32 having reinforcing fibers 34that are oriented generally in parallel with respect to a predeterminedorientation direction 36. A second layer 38 includes reinforcing fibers40 oriented generally perpendicular to the reference direction 36. Athird layer 42 includes reinforcing fibers 44 oriented at an angle αwith respect to the reference direction 36, and a fourth layer 46 thatincludes reinforcing fibers 48 oriented at an angle −α with respect tothe reference direction 36. Although the angle α may be any angle thatis intermediate between the orientation of the reinforcing fibers 34 ofthe first layer 34, and the reinforcing fibers 40 of the second layer38, in a particular embodiment, the angle α is approximately aboutforty-five degrees.

Still referring to FIG. 2, the first layer 32, the second layer 38, thethird layer 42 and the fourth layer 46 may have any desired thicknesses,and the fibers 34, 40, 44 and 48 may be of any desired compositions, butin one particular embodiment, the reinforcing fibers 44, 48 and 40 inthe third layer 42, the fourth layer 46 and the second layer 38,respectively, are intermediate to high modulus carbon fibers, having amodulus of at least about 50 M-lbs. per square inch. One suitable carbonfiber is the M-40J carbon fibers available from Toray Carbon FibersAmerica, Incorporated, of Decatur, Ala., although other suitablealternatives are available. In the same particular embodiment, thereinforcing fibers 34 in the first layer 32 are generally high strengthcarbon fibers having a modulus of at least about 40 M-lbs. per squareinch, such as the T-800 carbon fibers, also available from Toray CarbonFibers America, Incorporated, although other suitable alternativesexist.

The first layer 32, the second layer 38, the third layer 42 and thefourth layer 46 of the ply arrangement 30 may be present in the skinmember 20 of FIG. 1 in any desired interleaved arrangement and in anyrelative proportion. For example, and in another particular embodiment,the skin member 20 includes approximately about ten percent of the firstlayer 32, approximately eighty percent of the third layer 42 and thefourth layer 46, and approximately about ten percent of the second layer38, where the stated percentages are with reference to the total numberof layers in the structure. In yet another particular embodiment, thethird layer 42 is approximately about forty percent of the skin member20, and the fourth layer 46 is approximately about forty percent of theskin member 20.

FIG. 3 is a schematic view of a ply arrangement 50 for the stringerportion 12 of FIG. 1, according to still another embodiment of theinvention. The ply arrangement 50 also includes a plurality ofinterleaved plies that further include generally parallel reinforcingfibers. The ply arrangement 50 includes a first layer 52 havingreinforcing fibers 54 that are oriented at an angle δ with respect tothe predetermined orientation direction 36. A second layer 58 includesreinforcing fibers 60 oriented at an angle −δ with respect to thereference direction 36. A third layer 62 includes reinforcing fibers 64oriented at an angle γ with respect to the reference direction 36, and afourth layer 66 that includes reinforcing fibers 68 oriented at an angle−γ with respect to the reference direction 36. The angles δ and −δ arerelatively shallow angles that range in magnitude from approximatelyabout zero degrees and approximately about twenty degrees. In aparticular embodiment, the angle δ is approximately about five degrees.The angles γ and −γ are, in contrast, relatively broad angles having amagnitude generally greater than the shallow angles. The broad angles γand −γ therefore have magnitudes that range from approximately aboutforty-five degrees and approximately about ninety degrees. In anotherparticular embodiment, the angle γ is approximately about sixty-fivedegrees.

The first layer 52, the second layer 58, the third layer 62 and thefourth layer 66 may have any desired thickness, and the fibers 54, 60,64 and 68 may be of any desired composition. In a particular embodiment,however, the reinforcing fibers 54, 60, 64 and 68 are generally highstrength carbon fibers having a modulus of at least about 40 M-lbs. persquare inch, such as the aforementioned T-800 carbon fibers, availablefrom Toray Carbon Fibers America, Incorporated. The first layer 52, thesecond layer 58, the third layer 62 and the fourth layer 66 of the plyarrangement 50 may be present in the stringer portion 20 of FIG. 1 inany desired interleaved arrangement and in any relative proportion. Forexample, and in another particular embodiment, the stringer portion 12includes approximately about eighty percent of the first layer 52 andthe second layer 58, and approximately twenty percent of the third layer62 and the fourth layer 66.

With reference still to FIG. 3, and now also to FIG. 4, a plyarrangement 70 according to yet another embodiment of the invention willnow be described. The ply arrangement 70 includes a first layer group 72that includes the first layer 52, the second layer 58 and the thirdlayer 62 of FIG. 3 in the ordered arrangement shown. A second layergroup 74 also includes the first layer 52 and the second layer 58 ofFIG. 3, and also includes the fourth layer 66. A third layer group 76also includes the first layer 52, the second layer 58, and the fourthlayer 66. In contrast to the second layer group 74, however, thepositions of the first layer 52 and the second layer 58 are juxtaposed.A fourth layer group 78 includes the first layer 52, the second layer 58and the third layer 66 in the ordered arrangement shown. The first layergroup 72, the second layer group 74, the third layer group 76 and thefourth layer group 78 may be repeated within the stringer portion 12(FIG. 1) as desired to generate a desired thickness in the stringerportion 12. Moreover, it is understood that the thickness of the firstlayer group 72, the second layer group 74, the third layer group 76 andthe fourth layer group 78 may have approximately equivalent individualthicknesses, or they may be independently varied. In a particularembodiment, the individual layers in the first layer group 72, thesecond layer group 74, the third layer group 76 and the fourth layergroup 78 may be formed using a polymer tape material, or “pre-preg”material, that includes a plurality of fiber reinforcement fibers.Suitable pre-preg materials are also available from Toray Carbon FibersAmerica, Incorporated.

FIG. 5 is a flowchart that shows a method 80 of making a compositestringer and skin structure according to still yet another embodiment ofthe invention. At block 82, a stringer portion is generated byrepetitively forming the first layer 52, the second layer 58, the thirdlayer 62 and the fourth layer 66, and bonding the layers 52, 58, 62 and66 as shown in FIG. 3. More specifically, the foregoing layers may alsobe formed according to the ply arrangement 70 shown in FIG. 4. At block84, a skin member is generated by repetitively forming the first layer32, the second layer 38, the third layer 42 and the fourth layer 46, andbonding the layers 32, 38, 42 and 46 as shown in FIG. 2. At block 86, atleast one of the stringer portion and the skin member may be cured, anda film adhesive may be interposed between the cured member and theuncured member of the stringer and skin structure. At block 86, the filmadhesive and possibly any other uncured member are then cured bysubjecting the stringer portion, the film adhesive and the skin memberto a predetermined thermal environment to bond stringer portion to theskin member. At block 90, apertures may be formed in the assembledstructure by drilling, abrasive jet cutting, or by other suitableprocesses. Fasteners are then installed in the apertures and torqued toa prescribed value.

Those skilled in the art will also readily recognize that the foregoingembodiments may be incorporated into a wide variety of differentsystems. Referring now in particular to FIG. 6, a side elevation view ofan aircraft 300 having one or more of the disclosed embodiments of thepresent invention is shown. The aircraft 300 generally includes avariety of components and subsystems known in the pertinent art, whichin the interest of brevity, will not be described in detail. Forexample, the aircraft 300 generally includes one or more propulsionunits 302 that are coupled to wing assemblies 304, or alternately, to afuselage 306 or even other portions of the aircraft 300. Additionally,the aircraft 300 also includes a tail assembly 308 and a landingassembly 310 coupled to the fuselage 306, and a flight control system312 (not shown in FIG. 6), as well as a plurality of other electrical,mechanical and electromechanical systems that cooperatively perform avariety of tasks necessary for the operation of the aircraft 300.

With reference still to FIG. 6, the aircraft 300 may include one or moreof the embodiments of the composite stringer and skin structure 314according to the present invention, which may be incorporated intovarious structural portions of the aircraft 300. For example, thevarious disclosed embodiments may be used to form structural portions inthe wing assemblies 304 and/or structural portions in the tail assembly308.

The aircraft 300 is generally representative of a commercial passengeraircraft, which may include, for example, the 737, 747, 757, 767 and 777commercial passenger aircraft available from The Boeing Company ofChicago, Ill. In alternate embodiments, the present invention may alsobe incorporated into flight vehicles of other types. Examples of suchflight vehicles include manned or unmanned military aircraft, rotarywing aircraft, or even ballistic flight vehicles, as illustrated morefully in various descriptive volumes, such as Jane's All The World'sAircraft, available from Jane's Information Group, Ltd. of Coulsdon,Surrey, UK.

While preferred and alternate embodiments of the invention have beenillustrated and described, as noted above, many changes can be madewithout departing from the spirit and scope of the invention.Accordingly, the scope of the invention is not limited by the disclosureof these preferred and alternate embodiments. Instead, the inventionshould be determined entirely by reference to the claims that follow.

1. A composite stringer and skin structure, comprising: a polymer-basedelongated stringer portion having reinforcing fibers positioned in aplurality of adjacent plies, a first portion of the reinforcing fibersbeing oriented at a relatively shallow angle relative to a selectedreference direction, and a second portion of the reinforcing fibersbeing oriented at a relatively broad angle relative to the selectedreference direction; a polymer-based and fiber-reinforced skin memberthat adjoins the stringer portion; and an adhesive material interposedbetween the stringer portion and the skin member.
 2. The compositestringer and skin structure of claim 1, wherein the first portion of thereinforcing fibers are oriented at angles that range in magnitudebetween approximately about zero degrees and approximately about twentydegrees, and the second portion of the reinforcing fibers are orientedat angles that range in magnitude between approximately about forty-fivedegrees and approximately about ninety degrees.
 3. The compositestringer and skin structure of claim 2, wherein the first portion of thereinforcing fibers are oriented at angles of approximately about fivedegrees, and the second portion of the reinforcing fibers are orientedat angles of approximately about sixty-five degrees.
 4. The compositestringer and skin structure of claim 1, wherein the stringer portion andthe skin member are comprised of reinforcement fibers having more thanone modulus.
 5. The composite stringer and skin structure of claim 1,wherein the stringer portion comprises a web portion that is orientedapproximately perpendicular to the skin member, and positioned between afirst flange portion and an opposing second flange portion thatsubstantially abuts the skin member.
 6. The composite stringer and skinstructure of claim 5, wherein at least one of a depth of the web portionand a respective width of the first and second flange portions variesalong a span of the structure.
 7. The composite stringer and skinstructure of claim 1, wherein the first layer and the second layercomprise approximately about eighty percent of the stringer portion andwherein the third layer and the fourth layer comprise approximatelyabout twenty percent of the stringer portion.
 8. The composite stringerand skin structure of claim 7, wherein the reinforcing fibers includecarbon fibers having a modulus of at least about 40 M-lbs. per squareinch.
 9. The composite stringer and skin structure of claim 1, whereinthe skin member comprises a first layer with reinforcing fiberssubstantially aligned with the selected reference direction, a secondlayer with reinforcing fibers oriented substantially perpendicular tothe selected reference direction, a third layer with reinforcing fibersoriented at an angle α relative to the reference direction and a fourthlayer with reinforcing fibers oriented at an angle −α relative to thereference direction.
 10. The composite stringer and skin structure ofclaim 7, wherein the angle α is approximately about forty-five degrees,and the reinforcing fibers in the second, third and fourth layersinclude fibers having a modulus of at least about 50 M-lbs. per squareinch.
 11. The composite stringer and skin structure of claim 9, whereinthe wherein the first layer comprises approximately about ten percent ofthe skin member, the second layer comprises approximately about tenpercent of the skin member, and the third layer and the fourth layercomprise approximately about eighty percent of the skin member.
 12. Thecomposite stringer and skin structure of claim 1, wherein the firstportion of the reinforcing fibers includes a first layer and a secondlayer, and the second portion includes a third layer and a fourth layer,further wherein the stringer portion further comprises a first layergroup, a second layer group, a third layer group and a fourth layergroup, wherein each layer group includes five adjacent layers selectedfrom the first layer, the second layer, the third layer and the fourthlayer.
 13. The composite stringer and skin structure of claim 12,wherein the first layer group and the fourth layer group include thethird layer interposed between a selected pair of the first and secondlayers.
 14. The composite stringer and skin structure of claim 12,wherein the second layer group and the third layer group includes thefourth layer interposed between a selected pair of the first and secondlayers.
 15. The composite stringer and skin structure of claim 1,further comprising at least one fastener that projects through aselected portion of the stringer portion and the skin member.
 16. Amethod of fabricating a stringer and skin structure, comprising: forminga stringer portion by repetitively bonding fiber-reinforced layers of apolymer material to impart a predetermined shape to the stringerportion; forming a skin member by repetitively bonding fiber reinforcedlayers of a polymer material to impart a predetermined shape to the skinmember; and adhesively joining the stringer portion to the skin member.17. The method of claim 16, wherein forming a stringer portion comprisesforming a plurality of adjacent plies that include a first portionhaving the reinforcing fibers oriented at a relatively shallow anglerelative to a selected reference direction, and a second portion havingthe reinforcing fibers oriented at a relatively broad angle relative tothe selected reference direction.
 18. The method of claim 17, whereinforming a plurality of adjacent plies further comprises orienting thereinforcing fibers in first the portion at angles that range inmagnitude between approximately about zero degrees and approximatelyabout twenty degrees, and orienting the reinforcing fibers in the secondportion at angles that range in magnitude between approximately aboutforty-five degrees and approximately about ninety degrees.
 19. Themethod of claim 16, wherein forming a skin member comprises forming afirst layer with reinforcing fibers substantially aligned with aselected reference direction, forming a second layer with reinforcingfibers oriented substantially perpendicular to the selected referencedirection, forming a third layer with reinforcing fibers oriented at anangle α relative to the reference direction and a fourth layer withreinforcing fibers oriented at an angle −α relative to the referencedirection.
 20. The method of claim 16, wherein adhesively joining thestringer portion to the skin member further comprises curing at leastone of the stringer portion and the skin member, interposing a filmadhesive between the stringer portion and the skin member, and curingthe film adhesive.
 21. The method of claim 16, wherein adhesivelyjoining the stringer portion to the skin member comprises interposing apaste adhesive between the stringer portion and the skin member.
 22. Themethod of claim 16, further comprising forming at least one aperturethat projects through a selected portion of the stringer portion and theskin member, inserting a fastener in the at least one aperture, andapplying a predetermined torque to the fastener to generate apredetermined compressive force.
 23. An aerospace vehicle, comprising: afuselage; wing assemblies and an empennage operatively coupled to thefuselage; and a composite stringer and skin structure positioned in atleast one of the wing assemblies and the empennage, the compositestringer and skin structure further comprising: a polymer-basedelongated stringer portion having reinforcing fibers positioned in aplurality of adjacent plies, a first portion of the reinforcing fibersbeing oriented at a relatively shallow angle relative to a selectedreference direction, and a second portion of the reinforcing fibersbeing oriented at a relatively broad angle relative to the selectedreference direction; a polymer-based and fiber reinforced skin memberthat adjoins the stringer portion; and an adhesive material interposedbetween the stringer portion and the skin member.
 24. The aerospacevehicle of claim 23, wherein the wherein the first portion of thereinforcing fibers are oriented at angles that range in magnitudebetween approximately about zero degrees and approximately about twentydegrees, and the second portion of the reinforcing fibers are orientedat angles that range in magnitude between approximately about forty-fivedegrees and approximately about ninety degrees.
 25. The aerospacevehicle of claim 23, wherein the skin member comprises a first layerwith reinforcing fibers substantially aligned with the selectedreference direction, a second layer with reinforcing fibers orientedsubstantially perpendicular to the selected reference direction, a thirdlayer with reinforcing fibers oriented at an angle α relative to thereference direction and a fourth layer with reinforcing fibers orientedat an angle −α relative to the reference direction.